The formation of the native copperores is not completely understood. The fact that it is a relatively uniquedeposit of a very large size makes the understanding of the ore formationprocesses problematic. The lack of sulfur in the ores is a very unusual featurein the formation of copper deposits. Most copper ore deposits will contain somenative copper, but is usually just found in the oxidized zone (gossan) of thedeposits. It is usually associated with cuprite, tenorite, and other secondarycopper minerals such as azurite, malachite, chrysocolla, etcetera. Often theywill be found as elongated spinel twins in gossans, but in the Lake Superiorores, this habit is rare. The two theories for the formation of the ore fluidsis that they either were associated with the final stages of the magmatism, orwere created by the burial metamorphism of the volcanic and sedimentary rocks.

     The ore formed approximately 30million years after the deposition of the Portage Lake Volcanics. This date wasobtained by radiometric dating (K-Ar) of adularia that is found associated withcopper in the amygdaloids in the rocks. The more permeable sections of the flowsand sedimentary rocks are highly altered. Most of the rocks were subjected toeither zeolite grade metamorphism (<200°C) or prehnite-pumpellyite grademetamorphism (200-400°C). The low permeability interior of the flows still havethe original igneous mineralogy. The economic copper deposits are usually found inassociation with quartz and epidote.

    The volcanic rocks had a relatively lowsulfur content since they were extrusive and most of the sulfur in the magmacould have been vented into the atmosphere. Subsequent leaching of copper (probably contained in low concentrations in the iron/titanium oxides) would have resulted in a sulfur poorfluid. The copper was most likely carried as a chloride complex in a (5-10%)Ca-Na brine. The deposition of copper from these ore fluids was most likelycaused by a combination of mixing ofthe copper-bearing fluids with meteoric water, reactions with the wall rock, and cooling of thesolutions. 

     There is a strong correlation withthe structure of the peninsula in the localization of economic copper deposits.The best ore grades were usually associated with faults that provided pathwaysfor fluids generated in the deeper rocks to escape, but limited the outwardmovement of the ore bearing fluids. The Allouez gap fault intersected thelargest number of ore bearing rocks ( up to 60% of the district'sproduction). 

Going from the tip of the Keweenaw to the southwest are fourareas of copper production. The mainarea of copper production is in the northeastern portion of the peninsula andruns from  Mohawk to Painesville. A very minor occurrence is south of thisarea near Winona. Another area of copper production was between Mass City andVictoria. Another major producing area was to the west in the White Pine area.

Geologic map of the major copper producing area. Majordeposits 1) Calumet & Hecla conglomerate; 2) Kearsarge amygdaloid; 3) Balticamygdaloid; 4) Quincy mine; 5) Osceola amygdaloid; 6) Isle Royale mine; 7)Atlantic mine. Heavy dotted green line is northern limit of abundant quartz inamygdaloids. After W. White "The native-copper deposits of northernMichigan" in "Ore deposits of the United States, 1933-67" AIME.

Copper "skull" 12.5 cm across. The skulls are copperdeposited around the cobbles in a conglomerate lode. The cobble is broken up andremoved, leaving the "skull". The conglomerate lodes were some of themost productive mines in the district.	Near the pathways for the copper bearing solutions, there wasextensive replacement of preexisting minerals. Usually, copper only filled inthe porosity of the rock, or sometimes the smaller clastic particles, but here,copper replaced even the pebble and cobble sized clastic grains ( longest direction is 6cm).

This is a 12cm piece of native copper. It filled a narrowfracture in the rock. It was bent during the mining operations. From the Centralmine dumps. The copper shows the blue/green oxidation to malachite or azurite.

     The White Pine deposit is similar tosome of the Keweenaw deposits as some native copper ( the second phase ofmineralization) was deposited at the sametime as the major copper deposits to the north. It was one of the few mines tosuccessfully exploit a sulfide ore. The Nonesuch shale contains copper sulfidesover much of the area in which it was deposited, but there were only economicconcentrations at the White Pine mine and in the Presque Isle syncline west ofthe Porcupine Mountains. The deposit was formed when pyrite near the base of theshale was replaced by chalcocite when copper bearing fluids circulated throughthe rocks in the closing stages of deposition of the Nonesuch shale (diagenesis- loss of water in the compaction of clays and sediments into rocks). The secondstage of mineralization occurred when the mid-continent rift was being closed and fluids from thelow grade metamorphism of the volcanic rocks circulated through the beds ( thesame age as the deposition of the majority of the native copper in the rest ofthe Keweenaw). Thefluids were introduced into the rocks through the thrust faults (White PineFault) Thismineralization consisted primarily of native copper and silver with subordinatechalcocite. Domeykite was also found in veins in this assemblage. The coppersulfides are concentrated within the shale beds, while native copper is morecommon in the upper beds of the Copper Harbor Conglomerate.

     Specimen of Nonesuch shale showing major replacementby native copper. The copper shows deformation of the sediment. Light greenishareas between native copper veins are primarily shale rock with minorreplacement of very fine grained native copper. Specimen is 6 cm. across.

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